Make sure to enable :

• Site Collection Administration->Site Collection Features->SharePoint Server Publishing Feature
• Site Actions->Manage Site Features->SharePoint Server Publishing

Upload custom CSS file to Alternate CSS URL .

• Upload your custom CSS file to Style Library or Site Assets
• Go to site setting > Look and Feel > Design Manager > Publish and Apply Design > Assign Master Page based on device channel
• At Alternate CSS URL > Specify a CSS file to be used by this site and all sites that inherit from it: > Browse your file.

/*custom css to disable SharePoint Logo, Divider and Sites Menu*/
o365cs-nav-header16 .o365cs-nav-o365Branding { display: none; }
.o365cs-nav-header16 .o365cs-nav-appTitleLine { display: none; }
.o365cs-nav-header16 .o365cs-nav-appTitle { display: none; }
[/code]</pre>
<pre>[code language="powershell"]
/*disbable if your Help button is not working */
#O365_MainLink_Help{display: none;}[/code]</pre>
<pre>[code language="powershell"]
/*disable NewsFeed, Onedrive and Sites Ribbon*/
.o365cs-nav-appTile[id='O365_AppTile_Newsfeed'],
.o365cs-nav-appTile[id='O365_AppTile_Documents'],
.o365cs-nav-appTile[id='O365_AppTile_Sites'] {
	display: none;
}

.o365cs-rsp-off.o365cs-newAL-off.o365cs-nav-navMenu {
	min-width: 0;